We hope to cast some light on the elusive problems of the mechanisms of thermal sigmatropic rearrangements and cycloadditions. Having now excluded a putative cyclo-hexen-1,4-diyl as a true intermediate in the [3,3]sigmatropic (Cope) rearrangement of a hex-1-en-5-yne, we propose to make a structural alteration in the rearrangement substrate that could lead to a selectively stabilized biradical intermediate and thereby could give the stepwise process a chance to reveal itself. The role of orbital overlap in controlling the stereochemical course at a remote site of the homodienyl and bis-homodienyl hydrogen shift is under study. A major objective is the development of a method for transfer of stereogenicity with efficiencies of a million-to-one. Similar objectives motivate the proposed study of the intramolecular ene cyclization of eneallenes. These studies have both theoretical and practical aims. The results will provide quantitative measures of the magnitude of overlap control and also will sharpen our understanding of concerted thermal reactions. The applications bear on health because of their impact on methods for the synthesis of medicinally important natural and artificial substances, including drugs and hormones.